I've literally no idea about 15mm.
I played the game in 28mm using just a white sheet and a couple of bits of white felt.Rather than the tile system. Just cut all the 'tile' areas down to 8"×8" and use a piece of 'L' shaped card to change direction and mark out the limits of the next 'tile'.The only reason for the tile size was the limits of my table top and the largest piece of scenery I intended on using.